1.6.8 Blobs

Nomenclature warning:

There are two different release() functions:

• The release() callback for a blob (see the definition of PL_blob_t).
• std::unique_ptr::release(), which passes ownership of a unique_ptr.

Disclaimer:

The blob API for C++ is not completely general, but is designed to make common use cases easy to write. For other use cases, the underlying C API can still be used. The use case is:

• The blob is defined as a subclass of PlBlob, which provides a number of fields and methods, of which a few can be overridden in the blob (notably: write_fields(), compare_fields(), save(), load(), and the destructor).
• The blob will not be subclassed.
• The blob contains the foreign object or a pointer to it (e.g., a database connection or a pointer to a database connection), plus optionally some other data.
• The blob is created by a predicate that makes the foreign object and stores it (or a pointer to it) within the blob - for example, making a connection to a database or compiling a regular expression into an internal form. This “create” predicate uses std::unique_ptr to manage the blob (that is, the blob is created using the new operator and is not created on the stack).
• Optionally, there can be a predicate that deletes the foreign object, such as a file or database connection close.
• The blob can be garbage collected, althought this might require calling the predicate that deletes the foreign object first. There is no provision for handling “weak references” (e.g., a separate lookup table or cache for the foreign objects).
• The blob must have a default constructor that sets all the fields to appropriate initial values.^{13This is used by the load() callback; the default implementation for a C++ blob is to throw an error.}
• The blob's constructor throws an exception and cleans up any resources if it cannot create the blob.^{14This is not a strong requirement, but the code is simpler if this style is used.}
• The foreign object can be deleted when the blob is deleted. That is, the foreign object is created using the new operator and passes ownership to the blob. More complex behavior is possible, using PlAtom::register_ref() and PlAtom::unregister_ref().
• The blob's lifetime is controlled by Prolog and its destructor is invoked when the blob is garbage collected. Optionally, the predicate that deletes the foreign object deletes the foreign object and the Prolog garbage collector only frees the blob.

A Prolog blob consists of five parts:

• A PL_blob_t structure that defines the callbacks. The PL_BLOB_DEFINITION() macro is typically used to create this, with the callbacks pointing to methods in the C++ blob.
• A structure that contains the blob data. This must have a constructor that references the PL_blob_t structure, and optionally a virtual destructor. The PL_BLOB_SIZE macro is used to define some required methods.
• A “create” or “open” predicate that unifies one of its arguments with a newly created blob that contains the foreign object. The blob is created using the new operator (not on the stack) and managed with std::unique_ptr.
• (Optionally) a “close” predicate that does the opposite of the “create” or “open” predicate.
• Predicates that manipulate the foreign object (e.g., for a file-like object, these could be read, write, seek, etc.).

For the PL_blob_t structure, the C++ API provides the PL_BLOB_DEFINITION(blob_class,blob_name) macro, which references a set of template functions that allow easily setting up the callbacks. The C interface allows more flexibility by allowing some of the callbacks to default; however, the C++ API for blobs provides suitable callbacks for all of them, using the PL_BLOB_DEFINITION() macro.

For the data, which is subclassed from PlBlob, the programmer defines the various fields, a constructor that initializes them, and a destructor. Optionally, override methods can be defined for one of more of the methods PlBlob::compare_fields(), PlBlob::write_fields(), PlBlob::save(), PlBlob::load(), PlBlob::pre_delete(). More details on these are given later.

There is a mismatch between how Prolog does memory management (and garbage collection) and how C++ does it. In particular, Prolog assumes that cleanup will be done in the release() callback function associated with the blob whereas C++ typically does cleanup in a destructor. The blob interface gets around this mismatch by providing a default release() callback that assumes that the blob was created using PL_BLOB_NOCOPY and manages memory using a std::unique_ptr.^{15This release() function has nothing to do with std::unique_ptr::release().} More details on this are in section 1.6.8.1.

The C blob interface has a flag that determines how memory is managed: PL_BLOB_NOCOPY. The PL_BLOB_DEFINITION() macro sets this, so Prolog will call the C++ destructor when the blob is garbage collected. (This call is done indirectly, using a callback that is registeered with Prolog.)

The C++ API for blobs only supports blobs with PL_BLOB_NOCOPY.^{16The API can probably also support blobs with PL_BLOB_UNIQUE, but there seems to be little point in setting this flag for non-text blobs.}

1.6.8.1 A review of C++ features used by the API

Some slightly obscure features of C++ are used with PlBlob and ContextType, and can easily cause subtle bugs or memory leaks if not used carefully.

When a C++ object is created, its memory is allocated (either on the stack or on the heap using new), and the constructors are called in this order:

• the base class's constructor (possibly specified in the intialization list)
• the constructors for all the fields (possibly specified by an initial value and/or being in the initialization list)
• the object's constructor.

When the object is deleted (either by stack pop or the delete operator), the destructors are called in the reverse order.

There are special forms of the constructor for copying, moving, and assigning. The “copy constructor” has a signature Type(const Type& and is used when an object is created by copying, for example by assignment or passing the object on the stack in a function call. The “move constructor” has the signature Type(Type&& and is equivalent to the copy constructor for the new object followed by the destructor for the old object. (Assignment is usually allowed to default but can also be specified).

Currently, the copy and move constructors are not used, so it is best to explicitly mark them as not existing:

Type(const Type&) = delete;
Type(Type&&) = delete;
Type& operator =(const Type&) = delete;
Type& operator =(Type&&) = delete;

A constructor may throw an exception - good programming style is to not leave a “half constructed” object but to throw an exception. Destructors are not allowed to throw exceptions,^{17because the destructor might be invoked by another exception, and C++ has no mechanism for dealing with a second exception.} which complicates the API somewhat.

More details about constructors and destructors can be found in the FAQs for constructors and destructors.

Many classes or types have a constructor that simply assigns a default value (e.g., 0 for int) and the destructor does nothing. In particular, the destructor for a pointer does nothing, which can lead to memory leaks. To avoid memory leaks, the smart pointer std::unique_ptr^{18The name “unique” is to distinguish this from a “shared” pointer. A shared pointer can share ownership with multiple pointers and the pointed-to object is deleted only when all pointers to the object have been deleted. A unique pointer allows only a single pointer, so the pointed-to object is deleted when the unique pointer is deleted.} can be used, whose destructor deletes its managed object. Note that std::unique_ptr does not enforce single ownership; it merely makes single ownership easy to manage and it detects most common mistakes, for example by not having copy constructor or assignment operator.

For example, in the following, the implicit destructor for p does nothing, so there will be a memory leak when a Ex1 object is deleted:

class Ex1 {
public:
  Ex1() : p(new int) { }
  int *p;
};

To avoid a memory leak, the code could be changed to this:

class Ex1 {
public:
  Ex1() p(new int) { }
  ~Ex1() { delete p; }
  int *p;
};

but it is easier to do the following, where the destructor for std::unique_ptr will free the memory:

class Ex1 {
public:
  Ex1() p(new int) { }
  std::unique_ptr<int> p;
};

The same concept applies to objects that are created in code - if a C++ object is created using new, the programmer must manage when its destructor is called. In the following, if the call to data->validate() fails, there will be a memory leak:

MyData *foo(int some_value) {
  MyData *data = new MyData(...);
  data->some_field = some_value;
  if (! data->validate() )
    throw std::runtime_error("Failed to validate data");
  return data;
}

Ths could fixed by adding delete data before throwing the runtime_error; but this doesn't handle the situation of data->validate() throwing an exception (which would require a catch/throw). Instead, it's easiser to use std::unique_ptr, which takes care of every return or exception path:

MyData *foo(int some_value) {
  std::unique_ptr<MyData> data(new MyData(...));
  data->some_field = some_value;
  if (! data->validate() )
    throw std::runtime_error("Failed to validate data");
  return data.release(); // don't delete the new MyData
}

The destructor for std::unique_ptr will delete the data when it goes out of scope (in this case, by return or throw) unless the std::unique_ptr::release() method is called.^{19The call to unique_ptr<MYData>::release doesn't call the destructor; it can be called using std::unique_ptr::get_deleter().}

In the code above, the throw will cause the unique_ptr’s destructor to be called, which will free the data; but the data will not be freed in the return statement because of the unique_ptr::release(). Using this style, a pointer to data on the heap can be managed as easily as data on the stack. The current C++ API for blobs takes advantage of this - in particular, there are two methods for unifying a blob:

• PlTerm::unify_blob(const PlBlob* blob) - does no memory management
• PlTerm::unify_blob(std::unique_std<PlBlob>* blob) - if unification fails or raises an error, the memory is automatically freed; otherwise the memory's ownership is transferred to Prolog, which may garbage collect the blob by calling the blob's destructor. Note that this uses a pointer to the pointer, so that PlTerm::unify_blob() can modify it.

unique_ptr allows specifying the delete function. For example, the following can be used to manage memory created with PL_malloc():

  std::unique_ptr<void, decltype(&PL_free)> ptr(PL_malloc(...), &PL_free);

or, when memory is allocated within a PL_*() function (in this case, using the Plx_*() wrapper for PL_get_nchars()):

  size_t len;
  char *str = nullptr;
  Plx_get_nchars(t, &len, &str.get(), BUF_MALLOC|CVT_ALL|CVT_WRITEQ|CVT_VARIABLE|REP_UTF8|CVT_EXCEPTION);
  std::unique_ptr<char, decltype(&PL_free)> _str(str, &PL_free);

The current C++ API assumes that the C++ blob is allocated on the heap. If the programmer wishes to use the stack, they can use std::unique_ptr to automatically delete the object if an error is thrown - PlTerm::unify_blob(std::unique_ptr<PlBlob>*) prevents the automatic deletion if unification succeeds.

A unique_ptr needs a bit of care when it is passed as an argument. The unique_ptr::get() method can be used to get the “raw” pointer; the delete must not be used with this pointer. Or, the unique_ptr::release() method can be used to transfer ownership without calling the object's destructor.

Using unique_ptr::release() is a bit incovenient, so instead the unique_ptr can be passed as a pointer (or a reference). This does not create a new scope, so the pointer must be assigned to a local variable. For example, the code for unify_blob() is something like:

bool PlTerm::unify_blob(std::unique_ptr<PlBlob>* b) const
{ std::unique_ptr<PlBlob> blob(std::move(*b));
  if ( !unify_blob(blob.get()) )
    return false;
  (void)blob.release();
  return true;
}

The line declaration for blob uses the “move constructor” to set the value of a newly scoped variable (std::move(*b) is a cast, so unique_ptr’s move constructor is used). This has the same effect as calling b->reset(), so from this point on, b has the value nullptr.

Alternatively, the local unique_ptr could be set by

std::unique_ptr<PlBlob> blob(b->release());

or

std::unique_ptr<PlBlob> blob;
blob.swap(*b);

If the call to PlTerm::unify_blob() fails or throws an exception, the virtual destructor for blob is called. Otherwise, the call to blob.release() prevents the destructor from being called - Prolog now owns the blob object and can call its destructor when the garbage collector reclaims it.

1.6.8.2 How to define a blob using C++

TL;DR: Use PL_BLOB_DEFINITION() to define the blob with the flag PL_BLOB_NOCOPY and the default PlBlob wrappers; define your struct as a subclass of PlBlob with no copy constructor, move constructor, or assignment operator; create a blob using std::unique_ptr<PlBlob>(new ...), call PlTerm::unify_blob(). Optionally, define one or more of: compare_fields(), write_fields(), save(), load() methods (these are described after the sample code).

1.6.8.3 The life of a PlBlob

In this section, the blob is of type MyBlob, a subclass of PlBlob. (Example code is given in section 1.6.8.5) and section 1.6.8.7.

A blob is typically created by calling a predicate that does the following:

• Creates the blob using

  auto ref = std::unique_ptr<PlBlob>(new MyBlob>(...))}
        

  or

  auto ref = std::make_unique<MyBlob>(...);
        

• After the fields of the blob are filled in:

  return PlTerm::unify_blob(&ref);
        

  If unification fails or throws an exception, the object is automatically freed and its destructor is called.

  If make_unique() was used to create the pointer, you need to call PlTerm::unify_blob() as follows, because C++'s type inferencing can't figure out that this is a covariant type:

  std::unique_ptr<PlBlob> refb(ref.release());
  // refb now "owns" the ptr - from here on, ref == nullptr
  return A2.unify_blob(&refb);
        

  If unification succeeds, Prolog calls:

  • PlBlobV<MyBlob>acquire(), which calls
  • MyBlob::acquire(), which sets the field MyBlob::symbol_, which is usually accessed using the method MyBlob::symbol_term(). If this all succeeds, PlTerm::unify_blob(ref) calls ref->release() to pass ownership of the blob to Prolog (when the blob is eventually garbage collected, the blob's destructor will be called).

At this point, the blob is owned by Prolog and may be freed by its atom garbage collector, which will call the blob's destructor (if the blob shouldn't be deleted, it can override the the PlBlob::pre_delete() method to return false).

Whenever a predicate is called with the blob as an argument (e.g., as A1), the blob can be accessed by PlBlobv<MyBlob>::cast_check(A1.as_atom()).

Within a method, the Prolog blob can be accessed as a term (e.g., for constructing an error term) using the method MyBlob::symbol_term(). This field is initialized by the call to PlTerm::unify_blob(); if MyBlob::symbol_term() is called before a successful call to PlTerm::unify_blob(), MyBlob::symbol_term() returns a PlTerm_var.

When the atom garbage collector runs, it frees the blob by first calling the release() callback, which does delete, which calls the destructor MyBlob::~MyBlob(). Note that C++ destructors are not supposed to raise exception; they also should not cause a Prolog error, which could cause deadlock unless the real work is done in another thread.

Often it is desired to release the resources before the garbage collector runs. To do this, the programmer can provide a “close” predicate that is the inverse of the “open” predicate that created the blob. This typically has the same logic as the destructor, except that it can raise a Prolog error.

1.6.8.4 C++ exceptions and blobs

When a blob is used in the context of a PREDICATE() macro, it can raise a C++ exception (PlFail or PlException) and the PREDICATE() code will convert the exception to the appropriate Prolog failure or error; memory allocation exceptions are also handled.

Blobs have callbacks, which can run outside the context of a PREDICATE(). Their exception handling is as follows:

void PlBlob::acquire()

, which is called from PlBlobV<MyBlob>::acquire(), can throw a C++ exception. The programmer cannot override this.

int PlBlob::compare_fields(const PlBlob *_b)

, which is called from PlBlobV<MyBlob>::compare(), should not throw an exception. A Prolog error won't work as it uses “raw pointers” and thus a GC or stack shift triggered by creating the exception will upset the system.

bool PlBlob::write_fields(IOStream *s, int flags)

, which is called from PlBlobV<MyBlob>::write(), can throw an exception, just like code inside a PREDICATE(). In particular, you can wrap calls to Sfprintf() in PlCheckFail(), although the calling context will check for errors on the stream, so checking the Sfprintf() result isn't necessary.

void PlBlob::PlBlob::save(IOStream *fd)

can throw a C++ exception, including PlFail().

PlAtom PlBlob::PlBlob::load(IOSTREAM *fd)

can throw a C++ exception, which is converted to a return value of PlAtom::null, which is interpreted by Prolog as failure.

bool PlBlob::PlBlob::pre_delete()

, which is called from PlBLobV<MyBLOB>::release(), can return false (or throw a PlException or PlExceptinFailBase, which will be interpreted as a return value of false), resulting in the blob not being garbage collected, and the destructor not being called. Note that this doesn't work well with final clean-up atom garbage collection, which disregards the return value and also doesn't respect the ordering of blob dependencies (e.g., if an iterator blob refers to a file-like blob, the file-like blob might be deleted before the iterator is deleted).

This code runs in the gc thread. The only PL_*() function that can safely be called are PL_unregister_atom() (which is what PlAtom::unregister_ref() calls).

1.6.8.5 Sample PlBlob code (connection to database)

Here is minimal sample code for creating a blob that owns a connection to a database. It has a single field (connection) and defines compare_fields() and write_fields().

A second sample code shows how to wrap a system pointer - section 1.6.8.7

struct MyConnection
{ std::string name;

  explicit MyConnection();
  explicit MyConnection(const std::string& _name);
  bool open();
  bool close() noexcept;
  void portray(PlStream& strm) const;
};

struct MyBlob;

static PL_blob_t my_blob = PL_BLOB_DEFINITION(MyBlob, "my_blob");

struct MyBlob : public PlBlob
{ std::unique_ptr<MyConnection> connection;

  explicit MyBlob()
    : PlBlob(&my_blob) { }

  explicit MyBlob(const std::string& connection_name)
    : PlBlob(&my_blob),
      connection(std::make_unique<MyConnection>(connection_name))
  { if ( !connection->open() )
      throw MyBlobError("my_blob_open_error");
  }

  PL_BLOB_SIZE

  ~MyBlob() noexcept
  { if ( !close() )
      Sdprintf("***ERROR: Close MyBlob failed: %s\n", name().c_str()); // Can't use PL_warning()
  }

  inline std::string
  name() const
  { return connection ? connection->name : "";
  }

  bool close() noexcept
  { if ( !connection )
      return true;
    bool rc = connection->close();
    connection.reset(); // Can be omitted, leaving deletion to ~MyBlob()
    return rc;
  }

  PlException MyBlobError(const char* error) const
  { return PlGeneralError(PlCompound(error, PlTermv(symbol_term())));
  }

  int compare_fields(const PlBlob* _b_data) const override
  { auto b_data = static_cast<const MyBlob*>(_b_data); // See note about cast
    return name().compare(b_data->name());
  }

  bool write_fields(IOSTREAM *s, int flags) const override
  { PlStream strm(s);
    strm.printf(",");
    return write_fields_only(strm);
  }

  bool write_fields_only(PlStream& strm) const
  { if ( connection )
      connection->portray(strm);
    else
      strm.printf("closed");
    return true;
  }

  bool portray(PlStream& strm) const
  { strm.printf("MyBlob(");
    write_fields_only(strm);
    strm.printf(")");
    return true;
  }
};

// %! create_my_blob(+Name: atom, -MyBlob) is semidet.
PREDICATE(create_my_blob, 2)
{ // Allocating the blob uses std::unique_ptr<MyBlob> so that it'll be
  // deleted if an error happens - the auto-deletion is disabled by
  // ref.release() inside unify_blob() before returning success.

  auto ref = std::unique_ptr<PlBlob>(new MyBlob(A1.as_atom().as_string()));
  return A2.unify_blob(&ref);
}

// %! close_my_blob(+MyBlob) is det.
// % Close the connection, silently succeeding if is already
// % closed; throw an exception if something goes wrong.
PREDICATE(close_my_blob, 1)
{ auto ref = PlBlobV<MyBlob>::cast_ex(A1, my_blob);
  if ( !ref->close() )
    throw ref->MyBlobError("my_blob_close_error");
  return true;
}

// %! portray_my_blob(+Stream, +MyBlob) is det.
// % Hook predicate for
// %   user:portray(MyBlob) :-
// %     blob(MyBlob, my_blob), !,
// %     portray_my_blob(current_output, MyBlob).
PREDICATE(portray_my_blob, 2)
{ auto ref = PlBlobV<MyBlob>::cast_ex(A2, my_blob);
  PlStream strm(A1, 0);
  return ref->portray(strm);
}

1.6.8.6 Discussion of the sample PlBlob code

• PL_BLOB_DEFINITION(MyBlob, "my_blob") creates a PL_blob_t structure with the wrapper functions and flags set to PL_BLOB_NOCOPY. It should be declared outside the PlBlob class and should not be marked const - otherwise, a runtime error can occur.^{20The cause of the runtime error is not clear, but possibly has to do with the order of initializing globals, which is unspecified for C++.}

• The MyBlob struct is a subclass of PlBlob. See below for a discussion of the default behaviors.
  • MyBlob contains a pointer to a MyConnection object and keeps a copy of the connection's name. The MyConnection object is handled by a std::unique_ptr smart pointer, so that it is automatically freed when the MyBlob object is freed.

  • A default constructor is defined - this is needed for the load() and save() methods; it invokes the PlBlob constructor.

  • The MyBlob class must not provide a copy or move constructor, nor an assignment operator (PlBlob has these as delete, so if you try to use one of these, you will get a compile-time error).

  • PlBlob’s constructor sets blob_t_ to a pointer to the my_blob definition. This is used for run-time consistency checking by the various callback functions and for constructing error terms (see PlBlob::symbol_term()).

  • PlBlob’s acquire() is called by PlBlobV<MyBlob>::acquire() and fills in the symbol_ field. MyBlob must not override this - it is not a virtual method. The symbol_ field can be accessed by PlBlob::symbol_term().

  • PlBlob::symbol_term() Creates a term from the blob, for use in error terms. It is always safe to use this; if the symbol hasn't been set (because acquire() hasn't been called), symbol_term() returns a “var” term - this can be checked with PlTerm::is_variable().

  • The MyBlob(connection_name) constructor creates a MyConnection object. If this fails, an exception is thrown. The constructor then calls MyConnection::open() and throws an exception if that fails. (The code would be similar if instead the constructor for MyConnection also did an open and threw an exception on failure.)

  • The PL_BLOB_SIZE is boilerplate that defines a blob_size_() method that is used when the blob is created.

  • The destructor MyBlob() is called when the blob is released by the garbage collector and in turn calls the MyBlob::close(), throwing away the result. If there is an error, a message is printed because there is no other way report the error. For this reason, it is preferred that the program explicitly calls the close_my_blob/1 predicate, which can raise an error. One way of doing this is by using the at_halt/1 hook.

  • The MyBlob::close() method is called by either the destructor or by the close_my_blob/1 predicate. Because it can be called by the garbage collector, which does not provide the usual environment and which may also be in a different thread, the only Prolog function that can be called is PlAtom::unregister_ref(); and the MyBlob::close() method must not throw an exception.^{21It isn't enough to just catch exceptions; for example, if the code throws PlUnknownError("..."), that will try to create a Prolog term, which will crash because the environment for creating terms is not available.} Because there is no mechanism for reporting an error, the destructor prints a message on failure (calling PL_warning() would cause a crash).

    PlBlob::close() calls MyConnection::close() and then frees the object. Error handling is left to the caller because of the possibility that this is called in the context of garbage collection. It is not necessary to free the MyConnection object here - if it is not freed, the std::unique_ptr<MyConnection>’s destructor would free it.

  • PlBlob::MyBlobError() is a convenience method for creating errror terms.

  • PlBlob::compare_fields() makes the blob comparison function more deterministic by comparing the name fields; if the names are the same, the comparison will be done by comparing the addresses of the blobs (which is the default behavior for blobs defined using the C API). PlBlob::compare_fields() is called by PlBlobV<PlBlob>::compare(), which provides the default comparison if PlBlob::compare_fields() returns 0 (``equal” ).

    The _b_data argument is of type const PlBlob* - this is cast to const MyBlob* using a static_cast. This is safe because Prolog guarantees that PlBlobV<PlBlob>::compare() will only be called if both blobs are of the same type.

  • PlBlob::write_fields() outputs the name and the status of the connection, in addition to the default of outputting the blob type and its address. This is for illustrative purposes only; an alternative is to have a my_blob_properties/2 predicate to provide the information.

    The flags argument is the same as given to PlBlobV<PlBlob>::write(), which is a bitwise or of zero or more of the PL_WRT_* flags that were passed in to the caling PL_write_term() (defined in SWI-Prolog.h). The flags do not have the PL_WRT_NEWLINE bit set, so it is safe to call PlTerm::write() and there is no need for writing a trailing newline.

    If anything in PlBlob::write_fields() throws a C++ exception, it will be caught by the calling PlBlobV<PlBlob>::write() and handled appropriately.

  • PlBlob::save() and PlBlob::load() are not defined, so the defaults are used - they throw an error on an attempt to save the blob (e.g., by using qsave_program/[1,2]).^{22The C API defaults would save the internal form of the blob, which is probably not what you want, so the C++ API throws an error as its default.}

• create_my_blob/2 predicate:
  • std::unique_ptr<PlBlob>() creates a MyBlob that is deleted when it goes out of scope. If an exception occurs between the creation of the blob or if the call to unify_blob() fails, the pointer will be automatically freed (and the MyBlob destructor will be called).

    PlTerm::unify_blob() is called with a pointer to a std::unique_ptr, which takes ownership of the object by calling std::unique_ptr<PlBlob>::release() and passes the pointer to Prolog, which then owns it. This also sets ref to nullptr, so any attempt to use ref after a call to PlTerm::unify_blob() will be an error.

    If you wish to create a MyBlob object instead of a PlBlob object, a slightly different form is used:

    auto ref = std::make_unique<MyBlob>(...);
      ...
    std::unique_ptr<PlBlob> refb(ref.release());
    PlCheckFail(A2.unify_blob(&refb));
    return true;
          

• close_my_blob/1 predicate:
  • The argument is turned into a MyBlob pointer using the PlBlobV<MyBlob>::cast_ex() function, which will throw a type_error if the argument isn't a blob of the expected type.

  • The MyBlob::close() method is called - if it fails, a Prolog error is thrown.

1.6.8.7 Sample PlBlob code (wrapping a pointer)

struct MyFileBlob;

static PL_blob_t my_file_blob = PL_BLOB_DEFINITION(MyFileBlob, "my_file_blob");

static const PlOptionsFlag<int>
MyFileBlob_options("MyFileBlob-options",
                   { {"absolute", PL_FILE_ABSOLUTE},
                     {"ospath",   PL_FILE_OSPATH},
                     {"search",   PL_FILE_SEARCH},
                     {"exist",    PL_FILE_EXIST},
                     {"read",     PL_FILE_READ},
                     {"write",    PL_FILE_WRITE},
                     {"execute",  PL_FILE_EXECUTE},
                     {"noerrors", PL_FILE_NOERRORS} });

struct MyFileBlob : public PlBlob
{ std::FILE* file_;

  std::string mode_;
  int flags_;
  std::string filename_;
  std::vector<char> buffer_; // used by read(), to avoid re-allocation

  explicit MyFileBlob()
    : PlBlob(&my_file_blob) { }

  explicit MyFileBlob(PlTerm filename, PlTerm mode, PlTerm flags)
    : PlBlob(&my_file_blob),
      mode_(mode.as_string())
  { flags_ = MyFileBlob_options.lookup_list(flags);
    filename_ = filename.get_file_name(flags_);
    file_ = fopen(filename_.c_str(), mode_.c_str());
    if ( !file_ ) // TODO: get error code (might not be existence error)
      throw PlExistenceError("my_file_blob_open", PlTerm_string(filename_));
    // for debugging:
    //   PlTerm_string(filename.as_string() + "\" => \"" +
    //                 filename_ + "\", \"" + mode_ +
    //                 ", flags=" + MyFileBlob_options.as_string(flags_) + "\")")
  }

  PL_BLOB_SIZE

  std::string read(size_t count)
  { assert(sizeof buffer_[0] == sizeof (char));
    assert(sizeof (char) == 1);

    buffer_.reserve(count);
    return std::string(buffer_.data(),
                       std::fread(buffer_.data(), sizeof buffer_[0], count, file_));
  }

  bool eof() const
  { return std::feof(file_);
  }

  bool error() const
  { return std::ferror(file_);
  }

  virtual ~MyFileBlob() noexcept
  { if ( !close() )
      // Can't use PL_warning()
      Sdprintf("***ERROR: Close MyFileBlob failed: (%s)\n", filename_.c_str());
  }

  bool close() noexcept
  { if ( !file_ )
      return true;
    int rc = std::fclose(file_);
    file_ = nullptr;
    return rc == 0;
  }

  PlException MyFileBlobError(const std::string error) const
  { return PlGeneralError(PlCompound(error, PlTermv(symbol_term())));
  }

  int compare_fields(const PlBlob* _b_data) const override
  { // dynamic_cast is safer than static_cast, but slower (see documentation)
    // It's used here for testing (the documentation has static_cast)
    auto b_data = dynamic_cast<const MyFileBlob*>(_b_data);
    return filename_.compare(b_data->filename_);
  }

  bool write_fields(IOSTREAM *s, int flags) const override
  { PlStream strm(s);
    strm.printf(",");
    return write_fields_only(strm);
  }

  bool write_fields_only(PlStream& strm) const
  { // For debugging:
    // strm.printf("%s mode=%s flags=%s", filename_.c_str(), mode_.c_str(),
    //             MyFileBlob_options.as_string(flags_).c_str());
    strm.printf("%s", filename_.c_str());
    if ( !file_ )
      strm.printf("-CLOSED");
    return true;
  }

  bool portray(PlStream& strm) const
  { strm.printf("MyFileBlob(");
    write_fields_only(strm);
    strm.printf(")");
    return true;
  }
};

PREDICATE(my_file_open, 4)
{ auto ref = std::unique_ptr<PlBlob>(new MyFileBlob(A2, A3, A4));
  return A1.unify_blob(&ref);
}

PREDICATE(my_file_close, 1)
{ auto ref = PlBlobV<MyFileBlob>::cast_ex(A1, my_file_blob);
  if ( !ref->close() ) // TODO: get the error code
    throw ref->MyFileBlobError("my_file_blob_close_error");
  return true;
}

1.6.8.8 Discussion of the sample PlBlob code (wrapping a pointer)

• This code provides a simple wrapper for some of the C “stdio” functions defined in <cstdio>. The blob wraps the file pointer returned from fopen() and also keeps a few other values for debugging (the mode, flags, filename from the call to fopen()) plus a buffer for read operations.

• A utility class‘PlOptionsFlag` is defined in fileSWI-cpp2-flags.h, for mapping a list of atoms to a bit-field flag. For example, the list [search,read] would map to‘examPL_FILE_SEARCH|PL_FILE_READ‘.

• The MyFileBlob struct defines the blob that wraps a FILE*. The constructor (which is called by predicate my_file_open/4) converts the flags term (a list of atoms or strings) to a flag that is passed to PL_get_file_name(), to convert the filename to a string containing the abslute file name. This is then passed to fopen(), together with the mode. If the call to fopen() fails, a C++ exception is thrown, to be handled by Prolog. Other errors, such as a wrong argument type to PL_get_file_name() can also cause an exception.

• MyFileBlob::read() ensures that the buffer is big enough and then calls‘fread()‘to return the buffer's contents.

• MyFileBlob::eof() and MyFileBlob::error() call feof() and ferror() respectively. They can be used to check the status of the call to MyFileBlob::read().

• The destructor calls MyFileBlob::close() and outputs a warning if it fails - a destructor is not allowed to throw a C++ exception, so this is the best we can do; it's better if the programmer explicitly closes the file rather than depending on the garbage collector to free the blob.

• MyFileBlob::close() calls fclose(). It then sets the FILE* to null, so that close won't be done twice.

• MyFileBlob::compare_fields(), MyFileBlob::write_fields(), MyFileBlob::write_fields_only(), MyFileBlob::portray() are similar to the same methods in MyBlob in section 1.6.8.5.

• Predicate my_file_open(File,Filename,Mode,Flags) calls the MyFileBlob constructor with Filename, Mode, flags and unifies the blob with File.

• Predicate my_file_close/1 calls MyFileBlob::close(), checks for an error and creates a Prolog error if the close failed.

1.6.8.9 Identifying blobs by atoms

Passing a Prolog blob around can be inconvenient; it is easier if a blob can be identified an atom. An example of this is with streams, which are identified by atoms such as user_input.

A utility class AtomMap is provided for this situation. See section 1.17.4.